Summary/Abstract The rapid evolution of influenza virus allows the virus to escape from protective humoral or cellular immune responses generated. Therefore, the induction of concerted immune responses including both strong B and T cell immunity against conserved influenza viral epitopes, are believed to be the key to provide broad and long-lasting immunity. However, the current understanding of the mechanisms and/or pathways that can simultaneously stimulate robust B and T cell immunity, particularly at the mucosal sites, are largely elusive. This likely represent a key ?bottleneck? for the development of ?universal vaccines? that can provide long- lasting and cross-protective immunity against different strains of influenza virus. We have recently identified that a population of lung CD4 helper T (TH) cells developed after influenza viral clearance, co-exhibiting follicular helper (TFH) and tissue-resident memory (TRM) cell features. Based on their gene expression, migration features and functional properties, we termed these cells as tissue-resident T helper cells (TRH). Importantly, TRH cells provide local help for the generation of strong germinal center B (BGC) and resident memory B (BRM) cell responses, as well as a CD8 TRM population that was shown to mediate protection against heterologous influenza infection. These results raise an intriguing idea that the promotion of strong TRH responses will augment protective mucosal immunity against both homologous and heterologous viral re-challenge. We will test the ?proof of principle? of this idea following primary influenza infection and after mucosal immunization of a promising ?universal? vaccine candidate (Nanovax). Three specific aims are proposed. Aim 1: To unravel the mechanisms shaping TRH cell identity and regulome. Aim 2: To identify lung environmental cues modulating TRH cell development and/or maintenance. Aim 3: To determine the function of TRH cells in the protective immunity against IAV re-challenge. Our long-term goal is to unravel the cellular and molecular mechanisms by which long-term humoral and cellular memory responses are properly programmed and/or long-term maintained in the respiratory mucosal sites. Such studies, we believe, will significantly aid the design of future influenza therapeutics and/or promising mucosal vaccines that can provide long-lasting protection against broad spectrum influenza strains (i.e. ?universal? vaccines).